Radio frequency identification (RFID) is a ubiquitous wireless technology which allows objects to be identified automatically. An RFID tag is a small electronic device with an antenna and has a unique serial number. For some RFID applications and in the ALOHA-based anticollision algorithms, the number of tags in the system needs to be estimated. In Trans. Autom. Sci. Eng., vol 6, no. 1, pp. 9-15, Jan. 2009, Chen, a probabilistic method for tag estimation in ALOHA-based RFID systems was proposed, based on the maximum a posteriori probability. Although this approach is novel and useful, it has a mathematical error in modeling the problem. In this short paper, we address this problem and provide the correct probabilistic model for the ALOHA-based RFID systems. Some consequences of correcting the error in Trans. Autom. Sci. Eng., vol 6, no. 1, pp. 9-15, Jan. 2009, Chen, are discussed and the model is validated via simulation. Using the correct model, the performance of the ALOHA-based anticollision algorithm can be improved.